This invention relates to a hydrogen-absorbing alloy, a method of modifying the surface of the hydrogen-absorbing alloy, negative electrode for battery and an alkaline secondary battery.
Hydrogen-absorbing alloy is known as being capable of stably absorbing and storing hydrogen several ten thousands times (calculated as a gas under normal temperature and pressure) as much as of its own volume. Therefore, hydrogen-absorbing alloy is noticed as a promising material for safely and easily storing, keeping and transporting hydrogen as an energy source. Hydrogen-absorbing alloy is also studied for utilization in a chemical heat pump or compressor by making most of a difference in property between hydrogen-absorbing alloys, some of them being developed for actual use. Recently, the application of hydrogen-absorbing alloys to a metal hydride secondary battery (for example, nickel-hydrogen secondary battery) as an energy source by making use of hydrogen stored in a hydrogen-absorbing alloy, as well as an electrode material by making use of its high catalytic activity in the absorption and desorption reaction of the hydrogen-absorbing alloys has been extensively developed.
As evident from these facts, the hydrogen-absorbing alloy has many possibilities for various applications in view of its physical and chemical characteristics, so that the hydrogen-absorbing alloy is now considered as being one of important raw materials in future industrial.
The metal capable of absorbing hydrogen and constituting the hydrogen-absorbing alloy may be in the form of single substance which reacts exothermically with hydrogen, i.e., a metal element capable of forming a stable compound together with hydrogen (for example, platinum group elements, lanthanum group elements and alkaline earth elements); or in the form of an alloy comprising such a metal, as mentioned above, alloyed with another kind of metals. One of the advantages of the alloy resides in that the bonding strength between a metal and hydrogen can be suitably weakened so that not only the absorption reaction but also the desorption reaction can be performed comparatively easily. Second advantage of the alloy resides in that the absorption and desorption characteristics of the alloy with respect to the magnitude of hydrogen gas pressure required for the reaction (equilibrium pressure; plateau pressure), the extent of equilibrium region (plateau region), the change (flatness) of equilibrium pressure during the process of absorbing hydrogen and the like can be improved. Third advantage of the alloy resides in the improvement in chemical and physical stability.
The composition of the conventional hydrogen-absorbing alloy may be classified into the following types; i.e., (1) an AB5 type (for example, LaNi5, CaNi5); (2) an AB2 type (for example, MgZn2, ZrNi2); (3) an AB type (for example, TiNi, TiFe); (4) an A2B type (for example, Mg2Ni, Ca2Fe); and other types (for example, cluster), wherein A represents a metal element which is capable of exothermically reacting with hydrogen, and B, another kind of metal. Among them, LaNi5 of (1), a laves phase alloy belonging to (2) and some kinds of alloy belonging to (3) are capable of reacting with hydrogen at the normal temperature, and chemically stable so that they are extensively studied as a candidate for an electrode material of a secondary battery.
Whereas, the hydrogen-absorbing alloy belonging to (4) A2B type is accompanied with the following problems. Namely, the alloy strongly attract hydrogen so that hydrogen once absorbed therein can be hardly released. The absorption and desorption reaction thereof occurs only when the temperature thereof is raised up to a relatively high degree (about 200 to 300xc2x0 C.), and the rate of the reaction, if occurred, is slow. The chemical stability, in particular the stability in an aqueous solution, of the alloy is comparatively low. The alloy is generally very viscous and hard so that the working such as pulverization of it is very difficult. In view of these facts, the hydrogen-absorbing alloy of A2B type is rarely utilized except for the storage and transport of hydrogen in spite of its excellent capacity of absorbing hydrogen which is comparable to other types of hydrogen-absorbing alloy on the basis of volume and, if calculated on the basis of weight, two to several times as high as that of other types of hydrogen-absorbing alloy. Therefore, of these problems inherent to the hydrogen-absorbing alloy of A2B type as explained above are solved, it would be possible to expand the application of the alloy not only to the same fields as those of other types of hydrogen-absorbing alloy but also to a new field of utilization.
By the way, there have been reported a number of academic papers on the hydrogen-absorbing alloy of this (5) type. However, up to date, the report of practical use or testing for practical use is almost none.
Meanwhile, there is disclosed in Jpn. Pat. KOKAI Publication No. 6-76817 a magnesium-based hydrogen-absorbing alloy represented by a composition formula of Mg2xe2x88x92xNi1xe2x88x92yAyBx (wherein x is 0.1 to 1.5; y is 0.1 to 0.5; A represents an element selected from Sn, Sb and Bi; B represents an element selected from Li, Na, K and Al) such for example as Mg1.5Al0.5Ni0.7Sn0.3; or Mg1.8Al0.2Ni0.8Sn0.2. There is also disclosed in this publication that the hydrogen-absorbing alloy can be utilized as a negative electrode material of an alkali secondary battery. However, since this hydrogen-absorbing alloy disclosed in the publication is fundamentally of A2B type, the hydrogen-absorbing and desorbing property thereof in the normal temperature region is poor. Therefore, in order to make it possible to absorb and desorb hydrogen under normal temperature and pressure, the hydrogen-absorbing alloy is covered on the surface thereof with a nickel metal compound or a phosphorous compound as disclosed in the publication.
As explained above, the A2B type hydrogen-absorbing alloy has a feature distinct from other types of hydrogen-absorbing alloy in that it is light in weight, large in capacity and low in raw material cost since its composition is mainly consisted of alkaline earth metals and iron group elements. However, the A2B type hydrogen-absorbing alloy is accompanied with various problems as explained above.
Accordingly, an object of the present invention is to provide a hydrogen-absorbing alloy which is chemically stable, in particular in an aqueous solution, and can be easily pulverized by way of mechanical means.
Another object of this invention is to provide a hydrogen-absorbing alloy which is improved of its hydrogen-absorbing property, in particular the hydrogen-absorbing at room temperature.
Another object of this invention is to provide a method of modifying the surface activity of the hydrogen-absorbing alloy so that hydrogen can be easily and sufficiently absorbed by the hydrogen-absorbing alloy.
A further object of this invention is to provide a negative electrode for a secondary battery, which is excellent in stability during the electrode reaction, and to provide an alkali secondary battery having an improved charge/discharge cycle property.
A still further object of this invention is to develop a means to evaluate the deteriorating rate of a hydrogen-absorbing alloy containing magnesium, and, on the basis of this means, to provide a negative electrode suited for practical use, which is excellent in the reversibility and stability in the electrode reaction and an alkali secondary battery provided with such a negative electrode.
Namely, according to the present invention, there is provided a hydrogen-absorbing alloy containing an alloy represented by the following general formula (I):
Mg2M1yxe2x80x83xe2x80x83(I)
wherein M1 is at least one element selected (excluding Mg, elements which are capable of causing an exothermic reaction with hydrogen, Al and B) from elements which are incapable of causing an exothermic reaction with hydrogen; and y is defined as 1 less than yxe2x89xa60.5.
According to the present invention, there is further provided a hydrogen-absorbing alloy containing an alloy represented by the following general formula (II):
Mg2xe2x88x92xM2xM1yxe2x80x83xe2x80x83(II)
wherein M2 is at least one element selected (excluding Mg) from the group consisting of elements which are capable of causing an exothermic reaction with hydrogen, Al and B; M1 is at least one element selected (excluding Mg and M2) from elements which are incapable of causing an exothermic reaction with hydrogen; x is defined as 0 less than xxe2x89xa61.0; and y is defined as 1 less than yxe2x89xa62.5.
Further, according to the present invention, there is also provided a hydrogen-absorbing alloy containing an alloy represented by the following general formula (III):
M2xe2x88x92xM2xM1yxe2x80x83xe2x80x83(III)
wherein M is at least one element selected Be, Ca, Sr, Ba, Y, Ra, La, Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu, Ti, Zr, Hf, Pd and Pt; M2 is at least one element selected (excluding M) from the group consisting of elements which are capable of causing an exothermic reaction with hydrogen, Al and B; M1 is at least one element selected (excluding Mg and M2) from elements which are incapable of causing an exothermic reaction with hydrogen; x is defined as 0.01 less than xxe2x89xa61.0; and y is defined as 0.5 less than yxe2x89xa61.5.
Moreover, according to the present invention, there is further provided a method of modifying the surface of a hydrogen-absorbing alloy comprises a step of treating the hydrogen-absorbing alloy with an Rxe2x80x94X compound, wherein R represents alkyl, alkenyl, alkynyl, aryl or a substituted group thereof; X is a halogen element.
Still more, according to the present invention, there is further provided a hydrogen-absorbing alloy, wherein a half-width xcex94(2xcex8) of at least one peak out of peaks of three strongest lines to be obtained by an X-ray diffraction of the alloy using Cukxcex1-ray as a radiation source lies in the range of 0.2xc2x0xe2x89xa6xcex94(2xcex8)xe2x89xa650xc2x0.
Still more, according to the present invention, there is further provided a hydrogen-absorbing alloy which comprises 10% or more of magnesium, wherein an apparent half-width xcex94(2xcex81) of a peak in the vicinity of 20xc2x0 lines in the range of 0.3xc2x0xe2x89xa6xcex94(2xcex81)10xc2x0, or an apparent half-width xcex94(2xcex82) of a peak in the vicinity of 40xc2x0 lies in the range of 0.3xc2x0xe2x89xa6xcex94(2xcex82)xe2x89xa610xc2x0 in an X-ray diffraction of the alloy using CuKxcex1-ray as a radiation source.
Moreover, according to the present invention, there is further provided a method of modifying the surface of a hydrogen-absorbing alloy comprising a step of mechanically treating the hydrogen-absorbing alloy under vacuum or in an atmosphere of an inert gas or hydrogen.
Moreover, according to the present invention, there is further provided a method of modifying the surface of a hydrogen-absorbing alloy, which comprising the steps of mechanically treating the hydrogen-absorbing alloy under vacuum or in an atmosphere of an inert gas or hydrogen.
Furthermore, according to the present invention, there is provide a negative electrode for battery containing a hydrogen-absorbing alloy comprising an alloy represented by the following general formula (I):
Mg2M1yxe2x80x83xe2x80x83(I)
wherein M1 is at least one element selected (excluding Mg, elements which are capable of causing an exothermic reaction with hydrogen, Al and B) from elements which are incapable of causing an exothermic reaction with hydrogen; and y is defined as 1 less than yxe2x89xa61.5.
According to the present invention, there is further provided an alkali secondary battery comprising a negative electrode containing a hydrogen-absorbing alloy comprising an alloy represented by the following general formula (I):
Mg2M1yxe2x80x83xe2x80x83(I)
wherein M1 is at least one element selected (excluding Mg, elements which are capable of causing an exothermic reaction with hydrogen, Al and B) from elements which are incapable of causing an exothermic reaction with hydrogen; and y is defined as 1 less than yxe2x89xa61.5.
According to the present invention, there is provided a negative electrode for battery containing a hydrogen-absorbing alloy comprising an alloy represented by the following general formula (II):
Mg2xe2x88x92xM2xM1yxe2x80x83xe2x80x83(II)
wherein M2 is at least one element selected (excluding Mg) from the group consisting of elements which are capable of causing an exothermic reaction with hydrogen, Al and B; M1 is at least one element selected (excluding Mg and M2) from elements which are incapable of causing an exothermic reaction with hydrogen; x is defined as 0 less than xxe2x89xa61.0; and y is defined as 1 less than yxe2x89xa62.5.
According to the present invention, there is further provided an alkali secondary battery comprising a negative electrode containing a hydrogen-absorbing alloy comprising an alloy represented by the following general formula (II):
Mg2xe2x88x92xM2xM1yxe2x80x83xe2x80x83(II)
wherein M2 is at least one element selected (excluding Mg) from the group consisting of elements which are capable of causing an exothermic reaction with hydrogen, Al and B; M1 is at least one element selected (excluding Mg and M2) from elements which are incapable of causing an exothermic reaction with hydrogen; x is defined as 0 less than xxe2x89xa61.0; and y is defined as 1 less than yxe2x89xa62.5.
According to the present invention, there is provided a negative electrode for battery containing a hydrogen-absorbing alloy, wherein a half-width xcex94(2xcex8) of a least one peak out of peaks of three strongest lines to be obtained by an X-ray diffraction using Cukxcex1-ray as a radiation source lies in the range of 0.2xc2x0xe2x89xa6xcex94(2xcex8)xe2x89xa650xc2x0.
According to the present invention, there is further provided an alkali secondary battery comprising a negative electrode containing a hydrogen-absorbing alloy, wherein a half-width xcex94(2xcex8) of a least one peak out of peaks of three strongest lines to be obtained by an X-ray diffraction using Cukxcex1-ray as a radiation source lies in the range of 0.2xc2x0xe2x89xa6xcex94(2xcex8)xe2x89xa650xc2x0.
According to the present invention, there is provided a negative electrode for battery containing a hydrogen-absorbing alloy comprising magnesium, wherein, when the negative electrode is immersed in a 6N to 8N aqueous solution of an alkali hydroxide, (a) either the elution rate of magnesium ion into the aqueous solution of alkali hydroxide of normal temperature is not more than 0.5 mg/kg alloy/hr, or the elution rate of magnesium ion into the aqueous solution of alkali hydroxide of 60xc2x0 C. is not more than 4 mg/kg alloy/hr, and (b) either the elution rate of component element of alloy into the aqueous solution of alkali hydroxide of normal temperature is not more than 1.5 mg/kg alloy/hr, or the elution rate of a component element of alloy into the aqueous solution of alkali hydroxide of 60xc2x0 C. is not more than 20 mg/kg alloy/hr.
According to the present invention, there is further provided an alkali secondary battery comprising a negative electrode accommodated in a case and containing a hydrogen-absorbing alloy comprising magnesium, a positive electrode accommodated in the case and so arranged s to opposite the negative electrode with a separator sandwiched therebetween, and an alkali electrolyte filled therein;
wherein a magnesium ion concentration in the alkali electrolyte 30 days or more after filling and sealing the alkali electrolyte in the case is not more than 2.2 mg/liter.
According to the present invention, there is further provided a hydrogen-absorbing alloy containing an alloy represented by the following general formula (V):
(Mg1xe2x88x92xM3x)20xe2x88x92yM4xe2x80x83xe2x80x83(V)
wherein M4 is at least one element selected Ni, Fe, Co, Cu, Zn, Sn and Si; M3 is at least one element selected (excluding the elements of M4) from the group consisting of elements which are more electronegative than Mg; x is defined as 0 less than x less than 0.5; and y is defined as 0xe2x89xa6y less than 18.
Further, according to the present invention, there is also provided a hydrogen-absorbing alloy containing an alloy represented by the following general formula (VI):
(Mg1xe2x88x92xM5x)20xe2x88x92yM6xe2x80x83xe2x80x83(VI)
wherein M5 is at least one element (excluding elements which are more electronegative than Mg) which has an atomic radius 1 to 1.5 times as high as that of Mg; M6 is at least one element selected Ni, Fe, Co, Cu, Zn, Sn and Si; x is defined as 0 less than x less than 0.5; and y is defined as 0xe2x89xa6y less than 18.
Moreover, according to the present invention, there is further provided a hydrogen-absorbing alloy which is formed of a mixture comprising:
an alloy having hydrogen-absorbing properties; and
at least one additive selected from the group consisting of (a) at least one element selected from Group IA elements, Group IIA elements, Group IIIA elements, Group IVA elements, VA elements, Group VIA elements, Group VIIA elements, Group VIIIA elements, Group IB elements, Group IIB elements, Group IIIB elements, Group IVB elements, Group VB elements and Group VIB elements; (b) an alloy formed of any combination of elements defined in the (a); and (c) an oxide of any of elements defined in the (a);
the mixture being mechanically treated under vacuum or in an atmosphere of an inert gas or hydrogen.
Moreover, according to the present invention, there is further provided a hydrogen-absorbing alloy, which comprises;
an alloy having hydrogen-absorbing properties; and
0.01 to 50% by volume of at least one powdered additive having 0.01 to 100 xcexcm in average diameter, which is dispersed in the alloy and selected from the group consisting of (a) at least one element selected from Group IA elements, Group IIA elements, Group IIIA elements, Group IVA elements, VA elements, Group VIA elements, Group VIIA elements, Group VIIIA elements, Group IB elements, Group IIB elements, Group IIIB elements, Group IVB elements, Group VB elements and Group VIB elements; (b) an alloy formed of any combination of elements defined in the (a); and (c) an oxide of any of elements defined in the (a).
According to the present invention, there is further provided an alkali secondary battery comprising a negative electrode containing a hydrogen-absorbing alloy comprising an alloy represented by the following general formula (V):
(Mg1xe2x88x92xM3x)20xe2x88x92yM4xe2x80x83xe2x80x83(V)
wherein M4 is at least one element selected Ni, Fe, Co, Cu, Zn, Sn and Si; M3 is at least one element selected (excluding the elements of M4) from the group consisting of elements which are more electronegative than Mg; x is defined as 0 less than x less than 0.5; and y is defined as 0xe2x89xa6y less than 18.
Further, according to the present invention, there is also provided an alkali secondary battery comprising a negative electrode containing a hydrogen-absorbing alloy comprising an alloy represented by the following general formula (VI):
(Mg1xe2x88x92xM5x)20xe2x88x92yM6xe2x80x83xe2x80x83(V)
wherein M5 is at least one element (excluding elements which are more electronegative than Mg) which has an atomic radius 1 to 1.5 times as high as that of Mg; M6 is at least one element selected Ni, Fe, Co, Cu, Zn, Sn and Si; x is defined as 0 less than x less than 0.5; and y is defined as 0xe2x89xa6y less than 18.
Moreover, according to the present invention, there is further provided an alkali secondary battery comprising a negative electrode containing a hydrogen-absorbing alloy which is formed of a mixture comprising:
an alloy having hydrogen-absorbing properties; and
at least one additive selected from the group consisting of (a) at least one element selected from Group IA elements, Group IIA elements, Group IIIA elements, Group IVA elements, VA elements, Group VIA elements, Group VIIA elements, Group VIIIA elements, Group IB elements, Group IIB elements, Group IIIB elements, Group IVB elements, Group VB elements and Group VIB elements; (b) an alloy formed of any combination of elements defined in the (a); and (c) an oxide of any of elements defined in the (a);
the mixture being mechanically treated under vacuum or in an atmosphere of an inert gas or hydrogen.
Moreover, according to the present invention, there is further provided an alkali secondary battery comprising a negative electrode containing a hydrogen-absorbing alloy, the hydrogen-absorbing alloy comprising:
an alloy having hydrogen-absorbing properties; and
0.01 to 50% by volume of at least one powdered additive 0.01 to 100 xcexcm in average diameter, which is dispersed in the alloy and selected from the group consisting of (a) at least one element selected from Group IA elements, Group IIA elements, Group IIIA elements, Group IVA elements, VA elements, Group VIA elements, Group VIIA elements, Group VIIIA elements, Group IB elements, Group IIB elements, Group IIIB elements, Group IVB elements, Group VB elements and Group VIB elements; (b) an alloy formed of any combination of elements defined in the (a); and (c) an oxide of any of elements defined in the (a).
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.